Gene interaction and the role of susceptibility-conferring genetic variants, or modifier loci, are important issues for current studies of complex genetic disease. Mutations in ion channel genes are a substantial cause of inherited disease. We developed a new model of bigenic disease susceptibility in the mouse, involving interaction between a splice site mutation in the sodium channel SCN8A and a defective allele of the modifier locus, SCNM1. On most strain backgrounds, this hypomorphic sodium channel mutation causes idiopathic dystonia, but the modifier gene mutation in strain C57BL/6J converts this to a lethal paralytic disease. The modifier gene encodes a U1 C-like zinc finger protein that is localized in the nucleus, suggestive of a direct role in 5' splice site selection. Expression of SCNM1 is widespread in vertebrate tissues. To investigate the function of this novel gene we will generate a null allele by homologous recombination, and carry out structure/function analysis in cultured cells. Proteins that interact with SCNM1 will be identified with the yeast two-hybrid system, TAP affinity purification, and confocal microscopy. Additional substrates for SCNM1 will be identified by comparing transcripts in tissues from normal and mutant mice. We will test SCNM1 as a candidate gene for other disease susceptibility traits of C57BL/6J mice. We will collaborate with the NIH Mouse Mutagenesis Centers to characterize new ENU-induced alleles of Scn8a. A small sensitized screen for dominant suppressors of sodium channel deficiency will be initiated in our laboratory. The proposed experiments will extend our understanding of the roles of SCNM1 and SCN8A in human disease. Because of the high level of alternative splicing in the nervous system, variation in SCNM1 may be of particular significance for neurological disease.